Sea turtle migration is the long-distance movement of sea turtles (superfamily Chelonioidea), comprising the swimming of adults to their breeding beaches, and also the offshore migration of hatchings. Sea turtle hatchings emerge from underground nests and crawl across the beach towards the sea. They then head offshore to the open sea. The feeding and nesting sites of adult sea turtles may be far apart, requiring some to migrate hundreds or even thousands of kilometres. Several patterns of adult migration have been identified. Some green sea turtles shuttle between nesting sites and coastal foraging areas. The loggerhead sea turtle uses a series of foraging sites. Others such as the leatherback sea turtle and olive ridley sea turtle do not keep to one coastal foraging site, but forage in different areas in the open sea. Although the leatherbacks seem to forage randomly, drifting passively with the currents, they still return to specific sites to breed. The ability of adult sea turtles to travel to precise locations has led biologists to wonder about their navigational mechanisms. Some have suggested that turtles might use the Earth's magnetic field to fix their position. There is evidence for this ability in juvenile green sea turtles.
Physiological and behavioral aspects of migration Sea turtles migrate up to 10,000 miles or more per year, traveling between breeding, foraging, and overwintering sites. Hatchlings migrate to open waters after emerging from their nest. Juvenile and adult sea turtles engage in seasonal migration, likely due to thermal variation and seeking areas with sufficient food. Sea turtles move north during spring and summer to more nutrient rich bodies of water. During fall and winter, they migrate back southward.
Sea turtles are considered ectothermic non-avian reptiles. Therefore, temperature has a major effect on both metabolic and physiological processes. Research has shown that during sea turtle migration, activity levels and VO2 within the turtles are higher than in rest. The size of the turtles also affects aerobic metabolism. A previous study indicated that as body size increased, so did the capacity for aerobic activity. The higher capacity for aerobic activity is effective when traveling long distances. The research team concluded that the migrations by sea turtles are helpful in regulating temperature, which increases their overall aerobic activity. The navigational methods of sea turtle migration help to increase the fitness of the sea turtle. The turtles use these cues to travel into deeper waters for a higher abundance of food and a lower risk of predation. For sea turtles who are endangered, finding an area of lower predation helps to maximize their overall fitness and maintain them as a species. For female sea turtles, returning to their natal beach to lay their offspring has been hypothesized to strengthen resistance to parasitic disease. This increases the fitness of the sea turtle along with its offspring.
Hatchling migration
Efficient movement of hatchlings away from the beach and shallow coastal waters is important in reducing the length of time that they are vulnerable to predators, which target the hatchlings on the beach or in shallow waters. Therefore, sea turtle hatchlings move offshore as an innate behaviour. The first part of the hatchling migration is called the 'frenzy period' which involves almost continuous swimming for the first 24–36 hours.
Orientation and navigation Studies of loggerhead and leatherback hatchlings have shown that moonlight reflected from the sea is an important visual cue in guiding movement from the beach to the sea. This navigational mechanism becomes a handicap if nesting sites are affected by artificial lighting since this can mean that hatchlings head towards the artificial lights rather than offshore towards the moonlit sea. Hence, the use of moonlight by turtle hatchings as a navigational cue can be considered an 'evolutionary trap'. Loggerhead and green turtles can detect the orbital movement of waves and use this information to swim perpendicular to the waves crests. This means they swim offshore, since close to the shore, wave crests run parallel to the beach. Further offshore the Earth's magnetic field is used to maintain an offshore direction and therefore head towards the open sea. The ability to head in a given direction without reference to landmarks, is called a compass mechanism and where magnetic cues are used to achieve this it is called a 'magnetic compass'. Hatchling loggerheads mature within the North Atlantic Gyre and it is important that they stay within this current system since here water temperatures are benign. It has been shown that loggerheads use the magnetic field to stay within the gyre. For example, when exposed to fields characteristic of a region at the edge of the gyre they responded by orienting in a direction which would keep them within the gyre. These responses are inherited rather than learned since the hatchlings tested were captured before reaching the ocean. Adult turtles may learn aspects of the magnetic field and use this to navigate in a learned rather than innate way.
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